Disable PDF handler switching

The UI configuration does not set any key in HKCU. Instead, changing the setting via the UI invokes the installer which sets the key in HKLM. The default application behavior varies depending on what is installed. A value of 1 disables the user's ability to change the default handler. Possible values include:
0 (or null): Allow the user to change the default viewer.
1: Don't allow the user to change the default viewer.